Method of removing silaceous cores from nickel and cobalt superalloy castings

ABSTRACT

The removal of silaceous ceramic cores from nickel- or cobaltbased superalloy casting is effectuated by heating the casting in a solution of potassium carbonate.

United States Patent [15] 3,698,467 Fassler et al; [451 Oct. 17, 1972 [54] METHOD OF REMOVING SILACEOUS [56] References Cited CORES FROM NICKEL AND COBALT SUPERALLOY CASTINGS UNITED STATES T T [72] Inventors: Michael H. Fa'ssler, Middletown; 1 gammond h g V P b V oungm 0th 3,617,747 11/1971 Wilkinson et al "164/132 x 1 Assignee= United c t p qm East Primary Eiaminer-J. Spencer Overholset Hartford Comb Assistant Exkzminer-John E Roethel 22 Filed: Jam 4 1971 Attorney-Owen J. Meegan 21 Appl -No; 103,544 [57] ABSTRACT The removal of 'silaceoiis ceralnfc cores from nickel- 52 US. Cl ..164/l32 or cobalt based superanoy casting is effectuated by [51] Int. Cl. .B22d 29/00 limiting the casting in a solution of potassium cap [58] Field of Search....l64/l32-; 23/312 R; 134/2, 19,

bonate.

3 Claims, 3 Drawing Figures I 1 METHOD OF REMOVING SILACEOUS CORES FROM NICKEL AND COBALT SUPERALLOY CASTINGS BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates toa method of removing silaceous ceramic cores from metal castings and particularly, to the removal of such cores from air cooling passages of gas turbine components which are formed of high temperature nickelor cobalt-base superalloys. In the manufacture of metal castings, it is frequently very difficult to completely remove the silaceous material forming the core without adversely affecting the chemical composition of the surface.

2. Descriptionof the Prior Art One of the more common ways to remove silaceous core material from castings has been to immerse the part in an aqueous caustic solution or a fused salt bath. Unfortunately, such treatments tended to oxidize some of the carbides that are located in the grain boundaries and in the matrix of the alloy, thereby exposing the surface to interand intragranular corrosion together with a depletion of the alloy constituents along the surface. Such corrosion markedly reduced the strength of the alloys. Also disclosed have been processes embodying the use of aqueous fluoride solutions which were effective in some controlled environments for certain types wherebyithe action of the acid on the metal can be regulated. Fluoboric acid dissolves the silica core thereby removing it from the metal casting. The process, however, can adversely affect the surface of the casting by altering its composition.

SUMMARY OF THE INVENTION In accordance with the present invention, the castings to be leached are immersed in an alkali metal carbonate solution at an elevated temperature, generally in excess of 200 F. The alkali metal carbonate solution is preferably saturated with the salt, but can be as low as w/o. Below about 25 w/o, insufficient alkali carbonate is provided to remove the silaceous core in a reasonable time. The temperature requirements are simply that the solution and the silaceous core must be sufficiently hot to leach the silaceous material in a reasonable amount of time. We have found that the solution should be above about 200 F preferably above boiling. Temperatures above 2l2 F. are realized by placing the component and the solution in a pressure vessel, commonly called an autoclave, having a heating element wrapped about the outside and arranged to raise the temperature to that which is desired. The time of the treatment is important insofar as the core is removed from the component when using the aqueous solution of alkali metal carbonate. We have found that potassium carbonate is highly desirable as the leachantbecause it effectively removes the silaceous material and does not adversely affect the surface of the component. Usually the silaceous material which forms the core is quartz or 2 fused silica and is essentially pure silica having a melting point above that of the casting.

BRIEF DESCRIPTION OF THE DRAWINGS face of superalloy castings in which cores have been removed by various solutions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In'FIG. .l, a 500X magnification of the surface. of a cobalt-base superalloy casting is shown. A core of silaceous material has been removed from the casting by dissolving it in a potassium hydroxide solution. FIG. 2 provides a 1,000X magnification of a'nickel-base superalloy casting which had the core removed by potassium hydroxide also. In both cases, it can be seen that deep fissures appear on the surface of the casting. It is believed that the potassium hydroxide not only reacts with the silaceous material of the core to dissolve it, but also reacts with carbides which were disposed near the surface of the alloy. The pockets of fissures on the surface are due to the carbides washing away because of 'the reaction with: the potassium hydroxide. In FIG. 2

there is also a depleted layer which is the result of the dissolution of alloy constituents by the leaching solution along the surface of the casting.

The rather long dark lines extending ,down from the surface of the'casting in FIGS. 1 and 2 are corrosion lines which weaken the surface of the alloy. We have also discovered that there is no effective way of removing residual alkali material when using an alkaline hydroxide as a leachant. Some alkali is entrapped in the corroded interstices or grain boundaries and on subsequent exposure to elevated temperatures in an oxidizing atmospheremore corrosion is generated.

In FIG. 3 of the drawings, a nickel-base superalloy component which has been treated according to the process of the present invention is shown at a 500x magnification. As-can be seen, neither pitting nor corrosion from the core removal treatment nor alloy depletion occurred.

As we have stated, cores from nickeland cobaltbase superalloy castings can be removed according tov the present process. Those alloys generally contain 5 to 25 w/o Cr, 5 to 15 w/o molybdenum, tantalum or tungsten, 2 to 8 w/o aluminum and titanium, less than 1 w/o carbide and the balance nickel and/or cobalt. Representative of the superalloys are those identified in the industry on the following Table I.

TABLE I 0.11 C, 4.3 Ta, 0.015 B, 0.07 Zr, balance Ni The present process can also be used with nickel-,

- specific embodiment of the present invention.

EXAMPLE 300 grams of 2were placed in a graduate and the graduate was then filled to 1,000 milliliters with water.

The contents were stirred until all of the 2was dissolved. 3-1 ,900, a nickel-based superalloy, was placed in an autoclave which was then filled with a p o r t ion of the pres/may desefibed-"samfion. The autoclave was heated to 425 F. for 12 hours. The sample was-then removed from the autoclave and washed. The silaceous material in the core had been leached and the surface was not appreciably affected.

In the following table a series of tests were performed using a 2leaching solution. 3-1 ,900 superalloy castings in various configurations, cast upon quarts cores, were used for the tests. The leaching rate is proportional to time, temperature, 2concentration and the shape of the casting.

tities of caustic hydroxides; placing a nickel-or cobalt 7 5 400-440 30 300-390 90% of core removed 8 5 415-445 30 300-400 of core removed 9 6 415-445 30 300-400 of core removed 10 6 490-520 30 650-800 of core removed 11 6 490-520 30 650-800 Core completelyv removed l2 6 420-450 30 300-390 40% ofcore removed 13 12 420-430 30 300-380 Core'completely Ev m I w removed As indicated, when the core was removed from'the casting there was no attack uponfthe surface of the alloy when using potassium carbonate solutions. The core removal is thus a function of the configuration of the casting, time, temperature, pressure, and concentration. Any of these can be modified to accomplish the removal in the shortest amount of time. V t

It is apparent that modifications and changes may be made within the spirit and scope of the present invention but it is our intention, however, only to be limited by the appended claims.

As our invention, we claim:

1. A method of removing silaceous cores from nickel or cobalt superalloy castings, the steps which comprise: forming an aqueous solution of alkali metal carbonate, said solution containing at least 25 weight percent alkali metal carbonate and being free of substantial quansuperalloy casting having a silaceous core in said solution and then heating to dissolve said core from said 1 casting. v

I 2. The process according to claim 1 wherein said alkali carbonate is potassium carbonate.

5 3. The process according to claim 1 wherein said solution is heated to above its boiling point in a pressure vessel. 

2. The process according to claim 1 wherein said alkali carbonate is potassium carbonate.
 3. The process according to claim 1 wherein said solution is heated to above its boiling poiNt in a pressure vessel. 